The most common and most likely answer to this crossword clue is the 5 letter word MURKY
I've seen this crossword clue in The Times
M
U
R
K
Y
Buddy explains!

The straight part
We know that "dark and /gloomy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dark and /gloomy = murky.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MURKY mean?
MURKY adjective- Hard to see through, as a fog or mist.
- Dark, dim, gloomy.
- Cloudy, indistinct, obscure.
- Dishonest, shady.